1. Field of the Invention
This invention relates generally to solenoid operated flow control valves and more particularly to multi-stage, solenoid actuated flow control valves having a pressure relief function.
2. Description of the Prior Art
Multi-stage flow control valves are usually employed in fluid systems operating at high fluid pressures. To operate a simple on/off flow valve in a high pressure environment would require a rather massive operating mechanism. For this reason, valves have been devised which operate in stages so that small operating forces can be used to control high pressure flow. In these types of valves, pressures in a first stage is released by an operating member, and the depletion of pressure in this stage, reduces a biasing force on a second stage primary valve member to effect its opening, thereby allowing fluid flow through the valve between an inlet and an outlet.
The first or pilot stage of the valve is typically operated by a solenoid. The primary valve member or piston, forming the second stage, is urged towards the open position by the fluid pressure at the inlet and urged towards a closed position, that is, a position interrupting flow between the inlet and the outlet, by a piston closure spring. Generally, a poppet in the pilot stage controls the application of biasing fluid pressure to the primary piston. When the poppet is seated, fluid pressure is applied to an effective pressure area on the piston and produces a force which balances the force applied to the inlet side of the piston by the incoming fluid pressure and thus, the closure spring will move and maintain the piston in its closed position. When the poppet opens, the balancing fluid pressure is released into the outlet, enabling the inlet fluid pressure to overcome the spring force, causing the piston to move to its open position and allow communication between the inlet and outlet of the valve.
Generally, an armature is disposed within the valve body adjacent the poppet, so that solenoid-induced movement in the armature causes attendant movement in the poppet towards and away from its associated seat. In a normally open type valve, the energization of the solenoid seats the poppet to effect closure of the primary piston and interrupt flow between the inlet and the outlet. De-energizing the solenoid allows the poppet to unseat and exhaust the biasing pressure holding the primary piston in its closed position thereby allowing fluid flow between the inlet and the outlet.
More sophisticated versions of multi-stage flow control valves include a pressure relief function which often takes the form of a spring biased poppet. In this type of valve, a solenoid operated armature effects movement in a spring biased poppet, towards and away from its associated seat. In the normally open configuration of this type of valve, the spring biased poppet, which although driven to its closed position by the solenoid, can be opened by a predetermined pressure at the inlet of the valve.
This type of valve is very useful in high pressure hydraulic systems where it can serve two purposes. First, it can be used to dump hydraulic pressure by appropriate actuation of the solenoid. Secondly, it can operate as a regulating valve and maintain a relatively constant system pressure determined by the biasing force on the poppet.
Typically, a normally open pressure relief valve, includes an armature disposed within the valve body adjacent a poppet biasing spring. Energizing the solenoid causes attendant movement in the armature which drives or compresses the biasing spring against the poppet, causing it to engage its valve seat. When the fluid pressure on the poppet exceeds the forces applied to it by the armature and biasing spring, the poppet is forced open and fluid pressure is exhausted through the valve seat. As long as the solenoid remains energized, the poppet will re-engage its valve seat once the excessive fluid pressure has been exhausted.
Certain problems and shortcomings have been recognized in prior art solenoid operated pressure relief valves. Many prior art valves employ a poppet return spring between the poppet and its associated seat which urges the poppet away from the seat to provide a positive opening force. This spring force is normally much less than the poppet biasing force applied to the poppet whenever the solenoid is energized. Nevertheless, the actual poppet biasing closure force is then reduced by the oppositely acting poppet return spring. These competing spring forces often made precise valve adjustments impossible. Secondly, because the armature compresses the poppet biasing spring to effect poppet closure, the pressure relief setting is partially affected by the extent of armature movement. Thirdly, in many of the prior art valves, partial valve disassembly was necessary to adjust the poppet biasing spring.
The problems present in the prior suggested valves were recognized and an attempt was made to construct a valve free of the identified shortcomings. The valve included a poppet valve assembly comprising a sleeve slidably supported within the valve body, which carried: a poppet, a poppet biasing spring and a biasing spring adjustment nut. Unlike the prior suggested valves, the armature effected movement in the poppet valve assembly and did not act directly against the poppet biasing spring. This configuration allowed precise control of the biasing force on the poppet and made its operation substantially independent of the poppet return spring and the extent of armature movement. In this valve, the armature moved the entire poppet assembly to effect poppet closure. This arrangement isolated the poppet biasing spring so that it was not affected by the forces of either the armature or the poppet assembly return spring.
This valve also included an external adjusting stem which was coupled to the biasing spring adjusting nut so that adjustments could be made to the poppet biasing spring without requiring valve disassembly.
Although this valve was partially successful in solving the problems of the prior art valves certain problems still remained. It was found, that frequently the movement in the poppet carrier was inhibited by the external adjustment coupling resulting in valve instability or inoperativeness. It was also found, that unauthorized access to the interior of the valve could be easily accomplished and it was desirable to have a more tamperproof valve. Finally, a provision for manually actuating the valve in the absence of electrical power was deemed desirable to facilitate checking and installation of the valve.